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470                              CHEMICAL ENGINEERING
chart, which turns with it. A pen in a carriage can be moved across the chart by means of a long screw. One galvanometer is connected to the thermocouple in the potentiometer circuit and the other directly to the differential couple through an adjustable resistance. The drum is turned by hand to maintain a balance in the potentiometer circuit and with the other hand the pen is made to follow the deflections of the second galvanometer. This operation gives directly a temperature vs. temperature difference curve. The temperature coordinate is 20 in. long and the other is about 5 in. long. The periods of the galvanometers are sufficiently short to allow the heating and cooling to be done in less than 1 hr. The curve obtained is a continuous line which is slightly ragged due to manual operation, and is easily
translated into metallurgical terms.    It can be replotted into a curve of # vs. ——-— the so-called ''Derived Differential Curve" due to Rosenhain.1
Temperature Control.—The meaning of temperature control can be extended to cover not only the control of temperatures, but also the control of processes through a knowledge of the temperatures involved. In this sense it has a very wide interest. A list of the industries in which temperature control is used in one way or another would cover nearly the entire field of industry. This discussion is confined to the field of high temperatures.
Practically all pyrometric installations are designed as an aid in furnace or oven operation. Some of the factors which work difficulty in the regulation of furnaces, ovens, kilns, tanks, etc., are: (1) Inconstancy of heat supply. (2) Variation of internal absorption or generation of heat. (3) Variation of heat lost by radiation, convection, etc. (4) Unsteady supply or composition of material to be heat treated.
Each of these items is intimately connected with the temperature and temperature variations of the material being heat treated.
In the ordinary thermocouple installation, the couple is so mounted that the temperature of the furnace or the material within the furnace is obtained. For temperature control, however, it may be necessary to regulate the temperature at some different point, the temperature of which has a known or determinable significance and relation to the temperature at the point where the final control is required. For precise regulation the temperature should be directly controlled at a point where the effect of a variation in heat supply is registered with as little time lag as possible, and the temperature of this point may be so adjusted as to indirectly control the temperature of the material being heat treated. If a large time lag exists, a thermocouple may still indicate a low temperature for some time after the development of heat in the furnace walls is sufficient to produce a temperature much too high. In a similar manner the couple may read high for some time after the amount of heat in the furnace walls is insufficient. Hence, the temperature of the control couple oscillates about the correct temperature and the magnitude of the departure from the desired steady state can be reduced to a minimum only by making the time lag as small as possible.
Automatic Alarm.—A galvanometer of low resistance may be fitted with electric contacts made by the pointer which will ring a bell. No relay is necessary. The alarm should be used only when the departure from a certain temperature range results in real danger to life or property, or when a process is completed. Otherwise it becomes a nuisance.
Manual Signaling.—This is of greatest use in extensive pyrometer installations having a central pyrometer station for indicators and recorders. Signals
1 Observations on Recalescence Curves.   Phys. Soc. Lond., 21, p. 180, 1908.